Complex signal synthesis involves combining an in-phase signal with a quadrature signal. The in-phase signal and the quadrature signal are modulated onto RF carrier signals having a ninety degree (90°) phase difference. Complex signals are widely used in modern communication systems because of their increased spectral efficiency over real component signals.
A digital-to-analog (D/A) converter takes a digital input and produces an analog output. D/A converters are especially useful in modern communication systems as microprocessors and other computers operate in the digital domain, but often need to communicate using analog signals. However, conventional D/A converters are limited in output frequency and bandwidth. Conventional D/A converters for complex signals are more than twice the size (and complexity) of D/A converters for non-complex signals.
Additionally, conventional D/A converters for complex signals suffer from false signals generated by the D/A converter process. These unwanted signals are known as image signals or aliased signals. For example, the D/A converter may generate image signals about the RF carrier during the up-conversion process, one signal being desired while the other signal being undesirable. Image frequency errors due to the aliased signals waste power on the unwanted image signal, thereby degrading signal strength of the desired signal and causing interference with other signals at the frequency of the unwanted, alias signal. RF filters can remove the unwanted image signals, but they are large and expensive, particularly for image signals having frequencies near the desired signal.